Same thing happened to me and I changed the radiator, then it all happened again! It was the intake manifold and head gaskets that were the problem all along. Get a pressure check done, or if you know how and have the equipment, do it yourself. Another way to check is if you see bubbles rising in your radiator while the engine is running and/or you have white smoke coming out of your tailpipe, plus you are losing coolant but can't find a leak anywhere - this means it is being sucked into the engine and being burnt off!
It sure is. If you have automatic trans. ,the cooling lines which run through the radiator maybe leaking into the radiator.It means a new radiator and possibly new tranny if you wait too long to get it fixed.
It's pretty serious. You'd better get it fixed before leaving it for long. And who put oil in your cooling bottle?